Making your theme semantically correct.

For a million reasons, a theme should be semantically correct. The better your semantics, the higher your search engine ranking is going to be, but speed of loading, accessibility, cross-device rendering, etc., are also important reasons to make sure your semantics are correct.

Headers

• h1: your site's title
• h2: the content title
• h3: any subtitles

So, on /node/123

• h1: flowers and beads
• h2: how to make hippie dresses
• h3: get stuff
• h3: put stuff together

But on /node or taxonomy/term/12 (a list of teasers)

• h1: flowers and beads
• h2: content OR h2: articles about rope
• h3: how to make hippie dresses
• h3: how to make hippie hats

And you should best make sure that even on /node/123 the h2 title is the link.

- menus: 99 to 1, your menu should be a list of links, styled
appropriately, not a bunch of <div> or | or etc, just a simple <ul>

- breadcrumbs: same thing, they should be a <ul> with appropriate styles

- content: content is content, it is <p>there is almost need to wrap these in endless <div> or <span> just to give them cool colors, just style the

directly in most cases, sometimes extra markup is needed but generally i see *too* much markup

definition lists: probably don't use them when you really should (think image galleries)

style external links and probably lots of other things that can be mentioned about semantic markup, some more useful articles here.